For example, Patent Document 1 discloses a conventional variable intake air mechanism. The variable intake air mechanism disclosed in Patent Document 1 includes a port switching valve enabled to switch between a long port and a short port, which are different in port length, for each of branches of an intake air manifold. For example, as shown in FIG. 7, the conventional port switching valve is controlled to use the long port in a CLOSE region, in which the engine is at a high load and at a low rotation speed, to harness pulsation to cause an inertia supercharging effect thereby to enhance an output power. The conventional port switching valve is further controlled to use the short port in an OPEN region other than the CLOSE region. More specifically, the conventional variable intake air mechanism throttles intake air in a region, in which the engine is at a low load less than a predetermined value THR1. In this low load region, the inertia supercharging effect caused by pulsation is not needed. Therefore, the conventional variable intake air mechanism uses the long port regularly without causing the port switching valve to switch the port length.
For example, Patent Document 2 discloses a conventional gas introduction mechanism. The conventional gas introduction mechanism disclosed in Patent Documents 2 has a gas passage, which distributes introduced gas such as EGR gas into each of branches of an intake air manifold. The conventional gas introduction mechanism is enabled to change an opening area of the gas passage for each branch thereby to reduce variation in introduction gas among cylinders of the engine.
(Patent Document 1)
Publication of unexamined Japanese patent application No. 2010-151062
(Patent Document 2)
Publication of unexamined Japanese patent application No. 2012-219626
It is noted that, in the conventional gas introduction mechanism disclosed in Patent Document 2, as the engine rotation speed changes, pulsation caused in intake air changes accordingly. Consequently, distribution of introduced gas into the cylinders changes accordingly. It is further noted that, the opening area of the gas passage to each branch is constant regardless of change in the rotation speed of the engine. Therefore, even though distribution of introduced gas is desirable in a certain operation state, the distribution may be exacerbated in a state other than the certain operation state when the rotation speed of the engine is at a different value.
FIG. 8 and FIG. 9 show a simulation result produced by a CAE analysis to verify variation in EGR gas flow among cylinders. As in FIG. 8, in a case where a short port is used in a low load region, variation among cylinders is within a band of 0.5% on either side when the engine rotation speed is at 1000 rpm or 2000 rpm. To the contrary, in the same case, variation among cylinders may be substantially on or beyond a band of 1.0% on either side when the engine rotation speed is at 3200 rpm or 4400 rpm.
To the contrary, as in FIG. 9, in a case where a long port is used in a low load region, variation among cylinders may be substantially on or beyond a band of 1.0% on either side when the engine rotation speed is at 2000 rpm. In the same case, variation among cylinders is within a band of 0.5% on either side when the engine rotation speed is at other rotation speeds than 2000 rpm. As described above, operation environment, such as differential pressure through the gas passage caused by pulsation may differ according to the engine rotation speed and the throttle position (engine load). Therefore, combination of the gas introduction mechanism of Patent Document 2 and the intake air manifold, which is equipped with the variable intake air mechanism of Patent Document 1, may not enable to reduce variation in introduced gas among the cylinders sufficiently.